Tehran Radio claimed Iran had advanced to within about four miles of Basra, Iraq's second largest city. Residents of Basra, contacted by telex from Kuwait, reported that the center of the city had been evacuated and that there was continuous Iranian shelling at a rate of about five to six incoming shells a minute. Military experts said Iran was still pouring fresh troops into Iraq.

Countering all this, the official Iraqi news agency said July 15 that Iraq had repulsed invading Iranian troops - ''Iraqi forces have succeeded in cleansing the soil of the homeland of Iranian forces.''

Iran appears to be using the same unconventional - but in terms of human lives, costly - military tactics which proved successful in expelling Iraqi troops from Iranian territory earlier this year. Iranian television features footage every evening of bitter hand to hand combat between the Iranian invaders and Iraq's regular armed forces.

Iranian television crews accompany the human waves of thousands of volunteers who cross the border at night on motorcycles armed only with light weapons. These human waves advance straight into positions of the regular Iraqi armed forces, who, given their conventional military training, find it difficult to cope with Iran's unorthodox methods.

Military experts say that the Iranians are repeating methods worked out during the fighting on Iranian territory: advancing one day, digging themselves in during the following two days, then renewing their advance on the fourth day.

Iraq, meanwhile, reported that its planes had bombed economic targets in the Iranian cities of Eslamabad and Dehloran July 15. Reports from Iran say that the July 14 Iraqi bombing of three Iranian cities left its mark. Iraqi warplanes are said to fly high over Iranian cities dropping their lethal load at random on centers of civilian population.

Tehran radio has warned the residents of the Iranian capital, Baghdad, that Iraq may attack it, too. Military experts add that Iraq so far appears to be in control of the skies.

Tehran radio repeatedly states that the aims of the Iranian invasion are: the toppling of the regime of Iraqi President Saddam Hussein; and keeping Iraqi fire at a distance from Iranian cities.

But diplomats involved in attempts during the past two years to end the Gulf war peacefully believe that Iran launched its invasion of Iraq after realizing that the Iraqi defeat on Iranian territory was not sufficient to force the government of Saddam Hussein to pay war reparations.

These dipomats say that Iran wants to grab a piece of Iraqi territory as a bargaining chip to force Iraq to pay the $150 billion war reparation demanded by Iran.